ABB REC670 Applications Manual page 178

Section 8
Current protection
172
³ ×
I 1.2 3I
step1 0
EQUATION1201 V3 EN-US
The current setting for step 1 is chosen as the largest of the above calculated
residual currents, measured by the protection.
Step 2
This step has directional function and a short time delay, often about 0.4 s. Step 2
shall securely detect all earth faults on the line, not detected by step 1.
3I
0
IN
>
IEC05000154 V2 EN-US
Figure 74: Step 2, check of reach calculation
The residual current, out on the line, is calculated at an operational case with
minimal earth-fault current. The requirement that the whole line shall be covered
by step 2 can be formulated according to equation 42.
³ ×
I 0.7 3I (at remote busbar)
step2 0
EQUATION1202 V4 EN-US
To assure selectivity the current setting must be chosen so that step 2 does not
operate at step 2 for faults on the next line from the remote substation. Consider a
fault as shown in Figure 75.
1MRK 511 401-UEN A
One- or two-phase earth-fault
IEC05000154-en-2.vsd
Bay control REC670 2.2 IEC
Application manual
(Equation 41)
M15282-144 v7
(Equation 42)